Commercial marine operators have available to them generally three types of marine propellers. These include controllable pitch propellers in which blade pitch is continuously variable, solid propellers with fixed pitch, and various forms of built-up propellers with removable blades and/or adjustable pitch. Controllable pitch propellers are far superior operationally, and if they have any disadvantages, it would lie in the initial cost, which sometimes is a controlling factor, regardless of the favorable pay-out times arising from the improved operational efficiency. For most operations, the least desirable of the types of propellers are the fixed pitch solid propellers, the operating inefficiencies of which are well known, as is the substantial expense of repair. For instance, if a blade is damaged, the operator cannot remove the blade and replace it with a spare, but instead must change the entire propeller, at least during repair time. This is time consuming and costly. A further consideration is that many, if not most, of the river towboats used in inland waters have nozzles fitted around the propellers. When those propellers become damaged, they must pull the shaft and drop the rudders in order to change the propeller. This is a substantial job. At least relative to the fixed pitch solid propellers, river operators prefer the features of a built-up propeller since, if they damage one or two blades, they can remove those blades and replace them with spares. However, many of these built-up propellers have their blades attached through a bolted arrangement such that a blade must be displaced along its own longitudinal axis, i.e., radially with respect to the hub, in order to be removed or replaced, and with such an arrangement it is not possible to remove the blade when positioned within a nozzle, as is commonly the case with river towboats used on the Mississippi River and the Great Lakes, since there is insufficient tip clearance to permit lifting the blade out of the hub counterbore. We are aware of no adjustable pitch, built-up propeller in which the blades are removable without increasing the overall diameter of the propeller during removal, by virtue of having to move the blades radially outwardly to clear the hub counterbore. Perhaps this is one reason, possibly among others, that built-up propellers have not been adopted as widely as some of their advantages would seem to dictate. In our opinion, the industry needs, but the prior art has not provided, an adjustable pitch, built-up propeller which constitutes a meaningful compromise between the disadvantages of fixed pitch solid propellers and the operational advantages but high initial cost of controllable pitch propellers. Our invention proposes a design of adjustable pitch, built-up propellers which achieves this, and provides attractive economies of manufacture and operation.
As indicated above, built-up, adjustable pitch propellers are known in the art, exemplary ones being shown in U.S. Pat. No. 3,255,827 (Nichols) and U.S. Pat. No. 3,594,099 (Herbert). To our knowledge, these are subject to the disadvantage that their constructions do not permit blade removal in other than a radial direction from the hub, that is, such as to require diametral clearance around the blade tips, which clearance is not always available. It is also known in the art to provide built-up marine propellers in which the blades may be removed axially of the hub, such as not to require diametral clearance, although we cannot say that this was a recognized advantage in the prior art of which we are aware. Examples of earlier patents where axial removal of blades appears possible are U.S. Pat. Nos. 548,655 (Pagan), 787,745 (Freid), and 1,122,925 (Henrichsen). These prior art proposals are individually subject to one or more of various disadvantages, such as being not capable of pitch adjustment, being constructed such that selective removal of one or more blades without disturbing the other blades is not reasonably feasible, and simply incorporating designs and constructions that would be completely unrealistic for the power requirements of modern commercial marine vessels. For instance, the propellers of the Freid and Henrichsen patents are incapable of blade pitch change, along with their other obvious deficiencies. The propeller of the Pagan patent has a pitch change capability, but, in addition to being of highly unrealistic design and construction, is akin to a house of cards, in that removal of bolts sufficient to permit removal of one blade would cause collapse of substantially the entire assembly. The Pagan patent simply teaches nothing to the modern technology of commercial marine propeller design.
Although we do not consider it to constitute analogous technology, we have become aware of various rotary fan constructions wherein it is possible to effect non-radial removal of the fan blades and adjustment of their pitch. Exemplary of these constructions are U.S. Pat. Nos. 2,232,670 (Barrett) and 2,573,875 (Riddiford). We consider that their constructions and their teachings are unsuitable for use in marine propulsion systems. Additionally, blade replacement in these fan constructions while still mounted on the fan shaft would be at least extremely difficult, since the only structure retaining the blade mounting blocks on the periphery of the blade or hub plate is the cover plate, which must be removed in order to replace a blade. In this respect, these constructions are subject to the disadvantages of the previously mentioned Pagan patent.
In accordance with our invention, we provide an adjustable pitch, marine propeller in which the blades are removable generally axially of the hub. The hub is of split construction, having forward and aft sections in which a plurality of mating recesses are formed, fore and aft recesses cooperating to form housings for the blade roots or spindles. The recesses formed in the forward hub section partially receive the roots of the propeller blades such that, with the aft section removed, the blades are removable by axial displacement in a direction at least generally parallel to the longitudinal axis of the hub, and, with the clamping force of the aft section lessened, each blade is adjustable about its root axis so as to permit pitch adjustment. The engagement between the forward hub section recesses and the blade roots preferably is such that, under static conditions, a blade is supported by its forward recess until physically moved axially rearwardly. In our preferred construction, the aft hub section comprises a plurality of clamping segments, with each clamping segment having a recess formed therein cooperating with a recess in the forward hub section as mentioned above. In the assembled condition, the blade roots are clamped in their housings between the forward hub section and the clamping segments, such that pitch-changing rotation of the blades is prevented, but may be selectively permitted by lessening the clamping force. Preferably each clamping segment is removably mounted independently on the forward hub section, through an advantageous arrangement of bolts which we have found to be effective in accommodating the stresses and forces arising during operation.
In keeping with our preferred construction, the forward hub section has a radially recessed support section extending axially rearwardly, and the inner surface of each clamping segment is configured for mating engagement with a portion of the outer surface of the support section when the clamping sections are mounted. We prefer that the outer surface of the support section have a polygonal peripheral configuration when viewed axially of the longitudinal axis of the hub, such that a plurality of planar clamping segment engaging surfaces are defined, with each planar surface aligned with a corresponding forward hub section recess.
To minimize or restrain centrifugal strain generated by rotation of the propeller, we prefer that each clamping segment is formed with a radially recessed lip extending axially rearwardly, the outer surface of which lip defines a spigot surface. A retaining means, preferably in the form of a retaining ring, encompasses the spigot surfaces when the clamping sections are mounted, and preferably the lips of the clamping segments are configured such that the spigot surfaces form a continuous circular surface surrounded and embraced by a circular ring.
Other advantages and features, as well as objects, of our invention will become apparent from the ensuing description of a preferred embodiment, taken with reference to the appended drawings.